Digital automatic preset tyre inflation is now well recognized as being a common method and device for filling and regulating of automotive vehicle tyres throughout the world, since the early 1990's.
All such devices feature a controller, a processor, a pressure sensor, a visual display and a user interface for setting user-requested target pressures. A control algorithm enables the controller to regulate the gas to the required pressure. By use of a pre-settable pressure target, the processor allows efficient automatic inflation/deflation of the automotive tyre to the required setting.
In certain markets and applications it is advantageous to convert the air mixture contained within the tyre to a nitrogen rich mixture. It is well documented that filling tyres with a nitrogen rich mixture has advantages for the tyre and the motorist, these are:                dry nitrogen carries no water vapour, this provides cooler running avoiding the conversion of water to steam, as water vapour can vary the tyre pressure enormously;        reduction in tyre life due to premature ageing of the tyre carcass, through oxidisation and corrosion of steel reinforcements; and        improved retention of the tyre pressure, through a reduced rate of permeation through the tyre wall due increased nitrogen molecule size over oxygen. This process is slowed if the nitrogen mix is between 93% and 98% dependent upon the tyre pressure required over a conventional air mixture.        
During the nitrogen-for-air substitution process, the tyre(s), previously inflated with compressed air, are purged and re-inflated with a nitrogen rich gas source. This process of deflation and inflation is repeated until the tyre N2 mix lies in the 93% to 98% range previously referred to.
To minimise the cycle time for such purging operations, a single controller and sensor can be connected to a plurality of outlets i.e. air/gas hoses to the tyres, with the opportunity for venting the tyres with standard air mixture, the tyres in one system being in communication with one another via a distribution manifold.
However there are significant disadvantages in the above approach of utilising one pressure sensor in monitoring a multiple tyre application via an outlet tyre hose distribution manifold.
Firstly, the user cannot guarantee an equal tyre pressures due to the possibility of different flow rates through the tyre valves as the sensor can only measure an average pressure. With different flow rates through the tyre valves and also the possibility of different front to back tyre sizes, the accuracy of all tyres having the same pressure cannot be guaranteed. For example, to purge and fill four tyres to a target pressure of 30 psi, it is possible for the inflator to be in equilibrium at pressures of 33, 32, 28, 27 psi, as some of the tyres may fill more slowly than others. If the hoses are removed too soon from the tyre valves, then the pressures will be above or below the desired target.
To counter this occurrence a long balance delay must be allowed for in order to achieve the correct pressure equalization in all the tyre(s) and must be understood, and followed, by the user.
In markets with heavy regulation for tyre inflation equipments such as Germany this approach of one sensor being in communication with multiple tyres is not allowed.
Secondly, difficulties also arise in the detection of very minor leaks, in that ensuring equal tyre pressures becomes increasingly difficult when monitoring a shared tyre volume (with minor leaks) due to the total volume of all the cumulative tyres with leakage caused for example by incorrectly seated hose chucks.
Indeed, it is possible that one of the hose chucks can become detached and/or not connected securely to the tyre valve. This is made worse by the necessity for the hose chucks to be of normally of ‘closed end’ construction. Thus, the user may not be aware of the deficiency during the start or during the tyre purging and filling process, and may mean that one of the tyres has been improperly converted to the desired nitrogen purity.
Thirdly, a requirement for different front or rear tyre pressures, necessitates inflating all the tyres to one fixed pressure, with subsequent disconnecting of two hoses and re-inflating or deflating the two connected tyres to the new target pressure. This increases the total time to complete the nitrogen-for-air substitution time cycle.
Fourthly, removes the need for a manually operated relief valve to enable the process start and end the process to be terminated.
Fifthly, the tyre inflation process, when working with a plurality of tyres, must be started with all tyres at approximately the same pressure to ensure approximately equal tyre filling is achieved at the end of the process.